Process of pickling metals



LULI

Pinata May 21, 1940 UNITED STATES nxamme:

PATENT OFFICE PROCESS OF PIOKLING METALS Marion W. Harman, Nitro, W. Va, assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Application October 5, 1938, Serial No. 233,477

16 Claims.

of an acid nature.

Pickling solutions for removing oxides from the surface of metals ordinarily consist largely of a dilute solution of an inorganic acid such as sulfuric or hydrochloric acid. The acid although it dissolves the oxides and scale, also attacks the metal to some extent so that the acid is largely consumed and a certain amount of metal is lost. It also appears that the hydrogen liberated by the action of the acid on the metal has a serious embrittling effect upon the metal being treated. The escaping hydrogen also carries with it a fine spray of the acid liquor from the pickling bath which produces an atmosphere possessing corrosive properties and one which is injurious to health and damaging to equipment.

By the present invention, these objectionable features of the pickling process are substantially overcome or reduced to a minimum. More specifically, some of the objects of the present invention are to obviate over-pickling, embrittlement and pitting of the articles treated; to minimize the amount of metal 10st in pickling; to increase the efficiency of the pickling operation; to prolong the efiective life of the pickling bath; and particularly to facilitate the pickling operation. Other and further objects of the invention will be apparent from the following description and accompanying examples.

To accomplish to as great an extent as possible the objects set forth in accordance with the present invention, there is added to the pickling bath an inhibitor or regulating agent which acts to restrain the action of the acid from attacking clean metal but assists in removing the rust, scale, and other deposits or in other words the inhibitor causes the acid selectively to remove such materials without appreciably attacking the clean portions of the metal under treatment.

The new class of pickle regulators comprise the esters of thiocarbamic acids and are commonly referred to as thiourethanes. Except for the N-alkyl compounds which are liquids, they are usually solid crystalline compounds of relatively low melting point. One method for their preparation is by the action of ammonia or an amine on a xanthic ester although other methods have been described in the chemical literature and may be employed when desired. One precaution to be observed, whatever the method of preparation, is that the thiourethanes, while relatively stable compounds in other respects, readily undergo oxidation in the presence of mild oxidizing agents and should not be heated in the presence of oxidizing mineral acids nor in the presence of excess caustic soda which catalyzes their decomposition. The thiourethanes are a well known class of compounds and the present invention in nowise relates to their preparation nor is it limited thereto.

In order to further illustrate the class of com pounds to which the present invention pertains, the thiourethanes are designated by the following structural formula wherein R1 and R2 represent hydrogen, alkyl, aralkyl and carbocyclic groups, the latter term including typical aryl groups and hydrogenated aryl groups and R3 represents an ester forming radical such as alkyl, aralkyl and carbocyclic groups, the latter term likewise including typical aryl and hydrogenated aryl groups.

Typical examples of the preferred class of compounds are H N-bntyl thiourcthane --CiHI I-(?-OC,H;

N-dibutyl thiourethane -.(C4H9):=N(?00:H|

N-benzyl thiourcthane OI I(-OC 1H n N-phenyl thiourcthane .-O1!I(OC,H|

H N phenyl gahydroxy ethyl I thiouret no NOOC,H40H

H N-p-diphenyl thiourethane N-E-O-Cgfl;

As specific embodiments of the invention which are to be understood as illustrative of the invention and not limitative thereof, typical examples of the preferred class of materials were employed in the metal pickling process under conditions which duplicated, so far as possible those com-.

monly followed in commercial practice. A portion of one of the preferred class of compounds was incorporated in a pickling bath and its value as an inhibitor determined in the following manner: A test piece of 28 gauge hot rolled tin plate steel stock, approximately 4" x 3 in dimension, was immersed in the pickling bath comprising substantially 750 c. c. of a water solution containing approximately 6% by weight of 66 B. sulfuric acid and a small amount of one of the inhibitors above described. The temperature of the bath was maintained at substantially 82 C. The steel test piece after thoroughly cleaning was weighed before immersion in the bath, and after 40 minutes pickling the test piece was again weighed, the difference between the two weights representing the metal loss during pickling. The metal loss per square inch of surface exposed compared with the metal loss per square inch of surface exposed of a similar test piece treated in an analogous manner but without the use of any inhibitor provides a measure of the inhibiting value of the compound employed. The results obtained upon testing typical examples of the preferred class of materials are given in the following table. 0.234 gram of inhibitor was employed in testing thlourethane itself and in every other case 0.0936 gram of inhibitor was employed.

Table I Weight Weight Metal loss of metal of metal g per square mummiemployed before arm gig inch of test in test in surface grams grams gr exposed N- hcnyl thiouret ane 21.3808 21.3414 0.0394 .001641 N-cyclohexyl thiourethane 23%? 23.2114 0.0093 .000387 Thi0ui'eth8ne.- 24. 3387 24.2330 0.1057 .004404 N-phenyl a hydroxy ethyl thiourethane..- 22.9161 22.8864 0.0297 .001237 N-phenyl benzyl thioum ane 23.3690 23.3535 0.0155 .000645 N- henyl thiouree 22. 5100 22.4900 0.0200 .000833 N-phenyl butyl thiourethane 23.1478 23.1368 0.0110 .000458 N -a-naphthyl thiourethsne 23. 5199 2;.6098 0. 0101 .000420 Blank 22.2020 21.6383 0.5637 .0234

It is apparent from the data set forth that the preferred class of materials constitute an important and highly eflicacious group of inhibitors as in all cases wherein one of the preferred inhibiting compounds was incorporated in the pickle liquor there resulted a great saving in the loss in weight of the steel test strips as compared with that resulting when no inhibitor was employed.

Other modes of employing inhibitors of the class hereinbefore set forth in the acid pickling of metals are apparent to those skilled in the art to which the invention pertains. The products described may be employed in the presence of foaming agents, ionizable substances or other materials ordinarily used. Thus the preferred class of materials may be employed as the sole inhibitor or they may be employed in conjunction with other materials exhibiting an inhibiting action or with inert fillers, W etc. For example the pre erred c o materials were found to be particularly effective when employed in small quantities in conjunction with another known inhibitor and inert filling material whereby the preferred material although present as a minor proportion of the composition exhibited marked activating action so that the combined inhibiting action was superior to that of any ingredient alone. It is thus apparent that the preferred class of pickling inhibitors may be employed in a variety of useful ways and numerous modifications of the invention suggest themselves whereby improved results would be obtainable by employing the preferred thiourethanes either as the sole inhibitor, as an activator for other known inhibitors or in conjunction with non-inhibiting ingredients exhibiting other useful properties in the pickling bath. In addition the inhibitors of this invention are definite chemical compounds and are readily reproducible so that it is possible to attain uniformity of pickling action from bath to bath. Again, other non-oxidizing mineral acids than sulfuric acid may be employed in the pickle bath. Thus hydrochloric acid as well as other inorganic acids may be employed in the pickle bath in conjunction with the preferred class of inhibitors in the manner hereinbefore set forth.

What is claimed is:

1. A process of pickling a metal which comprises subjecting the metal to be pickled to the action of a mineral acid solution containing a small proportion of a thiourethane.

2. A process of picking a metal which comprises subjecting the metal to be pickled to the action of a mineral acid solution containing a small proportion of a compound possessing the structure R N 0-R, R: g

wherein R1 and R: are selected from the group consisting of hydrogen, alkvl, aralkyl and carbocyclic groups and R3 is an ester forming radical.

3. A process of pickling a metal which comprises subjecting the metal to be pickled to the action of a non-oxidizing mineral acid solution containing a small proportion of an N-carbocyclic thiourethane.

4. A process of pickling a metal which comprises subjecting the metal to be pickled to action of a non-oxidizing mineral acid solution containing a small proportion of an N-aryl thiourethane.

5. A process of pickling a metal which comprises subjecting the metal to be pickled to the action of a non-oxidizing mineral acid solution containing a small proportion of an N-phenyl thiourethane.

6. A process of pickling iron and steel products which comprises subjecting the metal to be pickled to the action of a sulfuric acid solution containing a small proportion of N-cyclohexyl thiourethane.

7. A process of pickling iron and steel products which comprises subjecting the metal to be pickled to the action of a sulfuric acid solution containing a small proportion of N-phenyl benzyl thiourethane.

8. A process of pickling iron and steel products 252. COMPOSITIONS,

ll. A pickling products which comprises a non-oxidizing mineral acid solution containing a small proportion of an N-carbocyclic thiourethane.

12. A pickling bath for metal products which comprises a non-oxidizing mineral acid solution containing a small proportion of an N-aryl thiourethane.

13. A pickling bath for metal products which comprises a non-oxidizing mineral acid solution ng a small proportion of an N-phenyl thiourethane.

14. A pickling bath for metal products which comprises a sulfuric acid solution containing a small proportion of N-cyclohexyl thiourethane.

15. A pickling bath for metal comprises a sulfuric acid solution containing a small proportion of N-phenyl benzyl th 16. A pickling bath for metal products which comprises a sulfuric acid solution con ng a small proportion of N-phenyl butyl thiourethane.

MARION W. AR-MAN.

Exam 

